The invention relates to methods and apparatus for milling openings in downhole structures in a wellbore.
To produce hydrocarbons from an underground formation or to inject fluids into an underground formation, wellbores are drilled through the earth subsurface to the desired formation. Such wellbores may be vertical, deviated, or horizontal wellbores. Wells may also be multilateral wells, which have multiple lateral branches that extend from a parent wellbore (also referred to as the main bore).
After a wellbore has been drilled into the earth subsurface, it is typically lined with casing or another type of liner. Casing extends from the well surface some distance into the wellbore. In some wells, liners are also used to line other portions of a wellbore.
In some cases, it may be desirable to change the trajectory of a wellbore after the wellbore has been drilled and the casing or liner has been cemented in the wellbore. The change in trajectory may be desired to reach better producing zones of a formation. Further, lateral branches may be extended from a cased or lined main bore to provide a multilateral well.
To change the trajectory of the wellbore or to add a lateral branch, windows are formed in the casing or liner to enable drilling of the lateral bore. The casing or liner window is generally cut by a milling assembly having one or more mills. The peripheral surfaces of the mills are generally covered with abrasive or cutting inserts made of a hard material, such as sintered tungsten carbide compounds braised on a steel mandrel. The mills are designed to cut through a steel casing or liner. A deflection tool, referred to as a whipstock, is generally set in the wellbore before the milling assembly is run into the wellbore. The whipstock is located in the proximity of the region in which the lateral bore is to begin. The whipstock provides a slanted surface that guides the mills of the milling assembly into the adjacent casing or liner. The whipstock pushes the milling assembly towards the casing or liner wall under action of a downward force on the milling assembly.
Although a whipstock is expected to support some milling damage, it may be difficult to predict how much whipstock material is left after milling has been performed. In addition, after milling operations have been completed, it may be difficult to retrieve the damaged whipstock, which can lead to a major obstruction of the well and subsequent abandonment of the section of the well below the whipstock. In addition, conventional milling assemblies may not provide adequate control of the window geometry. Further, using whipstocks for milling operations is also a time-consuming task since the whipstock is set in the wellbore ahead of the milling assembly, and after milling has been performed, the whipstock is retrieved.
Difficulties may also arise when using whipstocks in subsea milling operations. In subsea applications, a milling assembly is typically run from a sea vessel through a marine riser to wellhead equipment at the sea floor. The milling assembly is lowered into the subsea wellbore through the wellhead equipment and run to an interval where a whipstock is located. Unlike land wells, however, stability of the milling assembly is an issue due to up and down movement of the vessel. Such vertical movement may cause substantial damage to the milling assembly or to the whipstock that prevents further milling operations. One conventional solution is to employ a feeder control mechanism in the well head equipment to limit vertical movement of the milling assembly, which m ay occur due to motion of the sea vessel. However, this adds to the complexity of the wellhead equipment, which increases costs and reduces reliability.
A need thus exists for an improved method and apparatus for milling windows or other openings in well casings or liners or other downhole structures.
In general, according to one embodiment, a milling string for use in a wellbore having a first structure comprises a support assembly having a first end and a second end, at least one mill supported by the support assembly between the first and second ends, and a deflection apparatus attached to the support assembly to deflect the at least one mill to engage the first structure to form an opening in the structure.
Other features and embodiments will become apparent from the following description, from the drawings, and from the claims.